The present invention relates to a bar code for microlithographic reticles and more particularly, to a reticle identifying scheme which provides multiple levels of error checking.
In the manufacture of integrated circuit semiconductor devices, patterns are generated on the surface of a semiconductor substrate through a process which is known generally as microlithography. Typically the desired pattern is formed initially on a plate referred to as a reticle, e.g., the pattern is formed by photolithographically etching a layer of chrome on a glass substrate or carrier. Increasingly, the pattern on the reticle is projected at reduced scale onto the surface of the wafer although older systems often employed so-called contact or proximity printing. As multiple reticles may be used in the successive steps of manufacturing a given semiconductor device and there are many types of such devices, it is important that the reticles be carefully checked and identified prior to usage since the use of an incorrect reticle could destroy an entire run of semiconductor wafers which have already been subject to extensive earlier processing.
While it has previously been proposed to employ bar codes for identifying reticles, the codes used heretofore have been relatively simple and limited codes, and have neither utilized the unique characteristics of the reticle environment nor have they provided the exceptionally high degree of reliability which is desirable in automated semiconductor manufacturing lines.
As is understood by those skilled in the art, most bar code symbologies developed heretofore have been most concerned with the ability to read rapidly codes printed by common printing techniques. Among the problems encountered in such an environment are the tendency of inks to run and to accumulate. The ability to achieve an initial read was considered important in the design of such codes and significant, i.e. non-trivial error rates were typically acceptable. In keeping with these criteria, most bar codes employed relatively narrow ranges of bar widths and little or no error correcting capability.
In the case of the reticles used in semiconductor manufacturing, however, a converse situation exists. The images created by the chrome on glass etching process are typically of very high resolution and precision and also yield a high degree of contrast. The need of the system, i.e., the semiconductor manufacturing line, is also dissimilar in that any error at all can entail immense cost, since the use of an incorrect mask to process a run of partially manufactured semiconductor wafers would render useless and valueless the entire work product.
Among the several objects of the present invention may be noted the provision of a reticle bar code system of extremely high reliability; the provision of such a system in which provides several levels of error-checking; the provision of such a system which permits identifying a reticle at the point of usage of a reticle; the provision of such a system which permits the reading of bar code from a moving reticle; the provision of such a system which is highly flexible in operation; and the provision of such a system which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.